1. Field
The present disclosure relates generally to inspection systems and, in particular, to backscatter x-ray inspection systems. Still more particularly, the present disclosure relates to a method and apparatus for increasing the amount of backscatter detected off of an object.
2. Background
A backscatter x-ray inspection system is an example of a nondestructive inspection system that uses x-rays to inspect an object. Some currently available backscatter x-ray inspection systems include an x-ray tube, a collimator, and a detector. The x-ray tube generates and emits x-rays. The collimator filters these x-rays to form an x-ray beam using a portion of the x-rays that travel substantially parallel to a specified direction.
When the x-ray beam encounters the object, some or all of the x-rays in the x-ray beam are not absorbed by the object. Some or all of the x-rays may be scattered by the object. In particular, the x-rays may be scattered off of the surface of the object, the portion of the object under the surface of the object, or a combination of the two. The scattered x-rays are referred to as backscatter.
The detector detects some or all of this backscatter. The detected backscatter may be used to generate image data for the object. This image data may be used to form one or more images of the object. For example, the backscatter detected when the x-ray beam is directed at a particular location on the object may be used to generate an intensity value for a pixel in an image that corresponds to that particular location on the object.
The x-ray beam may be moved along the object in a selected pattern such as, for example, a raster pattern, such that the image data may be generated for different locations on the object. In one illustrative example, the direction in which the x-ray beam is pointed may be changed such that the angle of incidence of the x-ray beam, with respect to the object, changes. This image data is used to form one or more images of the object that may be used to determine whether any inconsistencies are present in the object.
Many currently used detectors for backscatter x-ray inspection systems include photomultiplier tubes with plastic scintillators. These types of detectors are typically located behind the x-ray tube. These detectors are configured to detect x-ray photons that are returned from a scattering effect of the x-ray beam directed toward the object being inspected.
The x-ray sources used for backscatter x-ray inspection systems may have various ranges of energy levels used to generate x-ray beams. For example, some sources of x-rays may have energy levels of up to around 400 keV. With these types of systems, x-ray photons in the backscatter may have an energy level from about 70 keV to about 450 keV. This range may be a spectrum of x-ray photons that may be present in the backscatter. The energy of the x-ray photons may depend on factors such as, for example, the scattering angle, Bremsstrahlung x-ray distribution, and other suitable factors.
The Bremsstrahlung x-ray distribution for x-ray photons has a spectrum that may vary from low energy at the low end of the spectrum to high energy at the high end of the spectrum. For example, an 80 keV beam may result in backscatter distributed in a spectrum from about 15 keV to about 80 keV.
This distribution of x-ray photons increases the difficulty in designing a detector for detecting backscatter x-ray photons as efficiently as desired. For example, the detector may more easily absorb x-ray photons in the middle of the spectrum. Lower energy photons in the spectrum may not penetrate the surface of the detector. Higher energy photons in the spectrum may travel all the way through the detector and may never be absorbed by the scintillator.
Currently, one manner in which this inefficiency is overcome is through increasing the size of the power supply for the x-ray source. The power supply may be increased such that sufficient x-ray photon flux to produce energy for x-ray photons in a desired range of the Bremsstrahlung x-ray distribution that can be detected by the detector may be produced. This larger power supply usually results in a larger x-ray tube. The increase in the size of the power supply and the x-ray tube increases the expense, size, and weight of the backscatter x-ray inspection system.
Therefore, it would be desirable to have a method and apparatus that takes into account at least some of the issues discussed above, as well as other possible issues.